Selective Catalytic Reduction (SCR) System
The SCR is an exhaust after treatment technology that treats exhaust gas.
The SCR (Selective Catalytic Reduction) assembly consists of the following components
- Diesel exhaust fluid (DEF) injector and SCR fluid tank
- Diesel exhaust fluid mixer
- SCR catalytic converter substrate
The SCR system is used to reduce levels of NOX (oxides of nitrogen) emissions.
Small quantities of Diesel Exhaust Fluid (DEF) are injected into the exhaust upstream of a catalyst when the system is within the correct operating temperature, where it vaporizes and decomposes to form ammonia and carbon dioxide. The ammonia is the desired product used in conjunction with the SCR catalyst, converts the NOX to harmless nitrogen (N2) and water (H2O).
DEF is also known as AdBlue®, AUS32, Chem 101, Bluetec and "Urea". DEF is an aqueous urea solution blend of 32.5% high purity urea and 67.5% deionized water.
DEF is stored in a separate DEF tank and is injected into the exhaust system using an electric pump (including level sensor).
It is possible for the DEF solution to form solid deposits within the exhaust. Small amounts of soft deposits can be normal within the exhaust and will be removed during a DPF regeneration. However, larger dry deposits can imply that the DEF injector is either injecting too much DEF, or operating outside of the normal operating temperature window. There should be no evidence of urea deposits outside of the exhaust system. Deposits outside of the exhaust indicate a leak.
A NOX sensor is mounted downstream of the catalyst to ensure that the system is reducing NOX output to the required level. Failure of the DEF injection system (including pump and control modules) or the catalyst could lead to reduced functionality of the system and trigger a diagnostic trouble code (DTC). Diagnosis of the DEF injection system should be completed prior to replacement of the catalyst.
The table below illustrates the approximate DEF usage for the given distances traveled under various driving conditions. The DEF usage may vary depending on:
- Driving style
- Trailer towing
- Loaded vehicle weight
- Weather
- Idle time
| Driving Style | Diesel Exhaust Fluid Range |
|---|---|
| Trailer towing /aggressive or city driving | 3000 km - 12000 km |
| Normal Driving | 12000 km - 16000 km |
| Steady Highway Driving | 16000 km - 18000 km + |
DEF may freeze in ambient temperatures below 12.2 deg.F (-11 deg.C). This may occur if the vehicle is not used for an extended period ant the ambient temperature is below 12.2 deg.F (-11 deg.C).
The vehicle is equipped with a DEF preheating system which allows DEF to operate when the ambient temperature falls below 12.2 deg.F (-11 deg.C).
Diesel Exhaust Fluid Level Warning Lamp
A warning lamp will illuminate and a message will appear in the information display when the DEF level is low and needs to be refilled.